Hydro-pneumatic power systems are used for generating power from the tidal or current motion of water in oceans, bays, and rivers. Typically, such systems require a high water head. However, a system has been developed to generate power using a lower water head. This system is described in U.S. Pat. Nos. 5,074,710 and 5,222,833, the disclosures of which are incorporated herein by reference.
This system uses a pair of chambers, each having upstream ingress and downstream egress ports sealable by a water gate. The water gates are cross-coupled on a common shaft such that the upstream ingress water gate of one chamber is coupled for synchronous movement with the downstream egress water gate of the other chamber. The sets of cross-coupled water gates open and close alternately in synchronism. Thus, when one set of gates is open, water enters one chamber from the upstream flow and exits the other chamber into the downstream flow. When the water levels in the chambers equilibrate with the upstream and downstream water levels, the open set of gates closes by a counterweight and the opposite set of gates opens under the force of the water level differential. Water begins filling the chamber with the low water level and emptying from the chamber with the high water level. Thus, a cycle of alternately filling and emptying the water chambers is maintained.
Each chamber contains an air space above the water surface. The chambers are interconnected to each other by a channel or duct through which the air may flow. As one set of cross-coupled gates opens, air pressure builds up in the chamber which fills with water and a partial vacuum builds up in the chamber which is emptying of water, thereby generating a flow of air through the channel from the filling chamber to the emptying chamber. When the open set of gates closes and the opposite set of gates opens, the build up of pressure and vacuum in the chambers is reversed. Thus, a flow of air through the channel in the opposite direction is generated. An air turbine connected to an electric generator is mounted in the channel to convert the energy of the flowing air to electric energy.
As stated, the air flow through the channel periodically reverses direction. Either complex valving to redirect the air flow into a single directional stream prior to driving the turbine or a unidirectional air turbine, that is, one capable of providing unidirectional rotation from bidirectional air flow, is required. Generally, three basic types of unidirectional reaction turbines are known, the Wells turbine, the McCormick turbine, and the Darrieus turbine.
The Wells reaction turbine comprises a series of rectangular airfoil-shaped blades arranged concentrically to extend from a rotatable shaft, as shown in FIG. 1. Typically, the turbine is mounted within a channel that directs the fluid flow linearly along the axis of the rotatable shaft. The blades are mounted to extend radially from the rotatable shaft and rotate in a plane perpendicular to the direction of fluid flow. Regardless of the direction in which the fluid flows, the blades rotate in the direction of the leading edge of the airfoils, which, in FIG. 1, is counterclockwise.
The Wells turbine is capable of rapid rotation. Its blades move substantially faster than the flowing air, causing high noise. Also, its efficiency is relatively low, first, because the rectangular blades create turbulence in the slower air, and second, because the effective surface area of the airfoil-shaped blades is limited to the outer tips, where the linear velocity is greatest. The blades cannot capture all of the available energy in the flowing fluid.
The McCormick turbine comprises a series of V-shaped rotor blades mounted concentrically between two series of stator blades, as shown in FIG. 2. The rotor blades are mounted for rotation in a plane perpendicular to the direction of fluid flow. The stator blades direct fluid flow to the rotor blades. To achieve unidirectional rotation with bidirectional fluid flow, the outer stator blades are open to fluid flowing from one direction, while the inner stator blades are open to fluid flowing from the opposite direction.
The McCormick turbine is more quiet and could be more efficient than the Wells turbine. However, its rotational speed is too slow for direct operation of an electric generator. Gearing is required to increase the speed up into the range of 1800 to 3600 rpm. Its configuration is also complex and expensive to manufacture.
The Darrieus machine is a reaction turbine with rectangular airfoil-shaped blades oriented transversely to the fluid flow and generally parallel to the axis of rotation, as shown in FIG. 3. The blades may be attached to the axis by circumferential end plates, struts, or by other known means. In some variations, the blades are curved to attach to the ends of the axis. A Darrieus reaction turbine having straight rectangular blades, mounted vertically in a rectangular channel, has been placed directly in a flowing body of water to harness hydropower. As with any reaction turbine, the rectangular blades of the Darrieus turbine rotate much faster than the fluid flow, causing turbulence in the fluid and lowering the efficiency of the turbine.
Thus, a need still exists for a quiet, efficient, simple, unidirectional reaction turbine that can operate at high speeds without gearing multiplication.